I've helped a friend of mine hack his Wii to use USB Loader GX off of a USB flash disk, and when we put a copy of Mario Kart Wii onto the flash disk and tried playing it using USB Loader GX and Waninkoko's cIOS251 rev16 and Hermes's cIOS38 rev4 (we tried using IOS38 as a base, IOS38+37, and IOS38+60). Usually, after you pick a cup in single player grand prix it freezes on a black screen before it loads the first race. Sometimes, however, it works.
As I've said we've tried all those cIOS's and they all do the same thing.